Multiplex links are employed to an ever-increasing degree to reduce the cost of cables used in communication networks.
CCITT Recommendation X22 defines an interface designed to be attached to a communication controller in which the received data and the transmitted data are multiplexed together.
In this type of link, as defined in Recommendation X22, six or eight bits are transferred on a given communication line within each time slot in each frame.
As the maximum bit rate allowed on the multiplex link is 48 kbps, the number of lines that can be attached thereto is dependent on the bit rates used on the individual lines.
In a data communication system, a conventional diagnostic test called a wrap test is performed to check the operation of the circuits associated with a given line. This consists in temporarily connecting the receive interface of the line to its transmit interface to permit comparing the received data with the original data.
However, a problem arises in the case of a multiplex link in that, when a wrap test is being performed on a particular line, the other lines also are in diagnostic wrap mode and, therefore, cannot be used to transfer data.
It is, therefore, an object of this invention to provide a device for performing a wrap test on one or more communication lines attached to a multiplex link without affecting the operation of the other lines attached thereto.